LAS VEGAS — In the days since Stephen Paddock killed 58 people and wounded hundreds more during a country music festival here, most of the attention has been on the roughly 10 minutes of horror as streams of bullets rained down from his perch on the 32nd floor of the Mandalay Bay Resort and Casino.

But new details announced by the Las Vegas Metropolitan Police Department on Monday have now turned the focus on six key minutes before Mr. Paddock fired his first shot on the festival crowd.

The Police Department had spoken of a “very heroic” security guard, Jesus Campos, who happened upon the 32nd floor after Mr. Paddock began shooting, on Oct. 1. It credited Mr. Campos with aiding the police and helping bystanders, after being wounded himself. In a startling shift, the department now says that Mr. Paddock had fired at the guard six minutes before he began firing on the crowd.

The announcement on Monday was the latest in a frustrating investigation that has flummoxed local and federal law enforcement, who have yet to uncover any information about the gunman’s motive. While police officials called the adjustments “minute changes” that are common in a complex investigation, the revelation raises new questions about the authorities’ response to the shooting and why Mr. Paddock stopped firing.

MGM Resorts, which owns Mandalay Bay, contested the police department’s announcement in a statement on Tuesday, saying that the company “cannot be certain about the most recent timeline” and that “what is currently being expressed may not be accurate.” It did not provide further details, citing the continuing investigation.

Here is what we know and what we don’t know about the police response to the shooting:

When did the security guard arrive and what was he responding to?

Mr. Campos arrived on the 32nd floor shortly before 10 p.m. to investigate an alarm indicating that a door to a room — not Mr. Paddock’s — was open. While he was there, Mr. Campos heard drilling noises coming from Mr. Paddock’s room. Sheriff Joseph Lombardo said Mr. Paddock was most likely using a drill to set up cameras or to place a rifle.

What did the guard do after he arrived?

At 9:59 p.m., six minutes before Mr. Paddock began shooting at the crowd, Mr. Campos was investigating the drilling noise when Mr. Paddock fired roughly 200 bullets through the door of his room. One responding police officer told “60 Minutes” that the door looked like “Swiss cheese” afterward.

It is unclear where in the hallway Mr. Campos was standing and how close to the door he was when he took the fire. Mr. Campos, who was wounded in the right thigh, retreated from the doorway and immediately told casino security personnel that he had been shot. When the police arrived, Mr. Campos directed officers to Mr. Paddock’s room and helped hotel guests evacuate until he was told he needed to seek medical attention himself.

Sheriff Lombardo said last week, before the timeline changed, that Mr. Campos’s “bravery was amazing.” When the chronology was adjusted on Monday, the sheriff stressed that Mr. Campos had immediately alerted his bosses about the gunman and then protected a maintenance worker who arrived on that floor. It is not known when and how the hotel security officials informed the police that Mr. Campos had been shot.

How quickly did the police respond to the shooting?

A group of eight officers gathered at Mandalay Bay and began climbing the stairs within minutes of the shots being fired at the concert, police officials said. Two officers arrived at the 31st floor at 10:12 p.m. and announced over their radio that the gunfire was coming from one floor above them.

The gunfire stopped three minutes later, according to the police timeline, and two officers arrived on the 32nd floor at 10:17 p.m. — roughly 18 minutes after Mr. Campos had been shot. Mr. Campos then told officers he had been shot and gave the exact location of Mr. Paddock’s room, the police said.

As eight more officers arrived, just before 10:30 p.m., they began clearing the hotel rooms, looking for any injured guests and watching for anyone assisting the gunman. Another team of eight officers arrived on the floor at 10:55 p.m. and the group breached Mr. Paddock’s room at 11:20 p.m., when they saw what one officer would later describe as an “armory” with roughly two dozen rifles.

Questions immediately arose about the police department’s response time and the 18 minutes between when Mr. Campos was shot and when the officers arrived on the floor.

“We’re talking about the 32nd floor of a massive hotel,” Todd R. Fasulo, an assistant sheriff, said in an interview on Tuesday. “It’s not like walking into a Walmart. You’ve got to get to the core of the hotel; you have to think about what you’re doing and account for safety.”

Could the police have responded differently?

Sheriff Lombardo and his top deputies have repeatedly praised officers who responded to the shooting, saying that they had acted “heroically” and did exactly what they were trained to do. The department will conduct an investigation to go over their response, Mr. Fasulo said.

The police do not know exactly how many officers responded to the shooting; many of them were off duty.

Mr. Fasulo said that, so far, “nothing gives me any indication that they did anything other than what they are trained to do, which is to formulate and execute a plan as quickly as they can.” He added that as the officers were forming their team, one turned to another and warned: “We don’t know what we’re getting into — there’s a possibility we’re going to take gunfire.”

“As a department we’re trained that we form up and we attack and we go after the threat,” Mr. Fasulo said. “We had multiple off-duty cops who instantly turned it on and did what they would do if they were on duty: run toward the shots.”

Why did the gunman stop shooting?

While the police know that Mr. Paddock shot himself in the head, they do not know precisely when he took his own life or why he stopped shooting.

“We probably will never know,” Mr. Fasulo said. “You can hypothesize all day long about that but unless we understand his head we can’t know for sure.”

A version of this article appears in print on , on Page A15 of the New York edition with the headline: Shift in Timeline Raises Questions About Police Response in Las Vegas. Order Reprints | Today’s Paper | Subscribe